infix to prefix without bracket

/****************************
 *  Umang B Bhatt            *
 *   bhatt.umang7@gmail.com  *
 *****************************/

/**
 * program for infix to prefix without bracket
 */

// the actual algorithm is quiet difficult so doing this simple way

/* OP:
 * Enter expression: a*b+d/e
 * prefix of entered (infix) expression is:
 * +*ab/de
 */

// I will be following the BSD style indentation in all the programs because it feels more readable to me


#include<stdio.h>
#include<string.h>

#define MAX 100

void main()
{

    int TOP = -1, PTOP = 0;
    char s[MAX];

    char pref[MAX] = {'\0'};

    int pre(char);

    int push(char[], int, char);
    int pop(char *, int *);

    char c[MAX] = {'\0'}, symb, x;

    char temp;

    int i, j;

    printf("Enter expression: ");
    gets(c);


    /*
     * here i don't have strrev function on my compiler (gcc) so i am doing this manually
     * but you can write strrev(c); if your compiler supports it
     */

    i = 0;
    j = strlen(c) - 1;

    while (i < j)
    {
        temp = c[i];

        c[i++] = c[j];
        c[j--] = temp;
    }

    /*
     * gets will give error when compiling on modern compiler
     */

    for (i = 0; c[i] != '\0'; i++) // NULL will give error on iso compiler

    {
        symb = c[i];
        if (isalpha(symb) > 0)
        {

            pref[PTOP] = symb;
            PTOP++;
        }
        else
        {

            while ((pre(symb))<(pre(s[TOP])) && TOP >= 0)
            {

                x = pop(s, &TOP);
                pref[PTOP] = x;

                PTOP++;
            }
            TOP = push(s, TOP, symb);
        }
    }

    while (TOP >= 0)
    {
        x = pop(s, &TOP);

        pref[PTOP] = x;
        PTOP++;
    }
    pref[PTOP] = '\0';


    //strrev(pref);
    // use strrev if it is there in your compiler
    // if not available then use following code
    i = 0;
    j = strlen(pref) - 1;



    while (i < j)
    {
        temp = pref[i];

        pref[i++] = pref[j];
        pref[j--] = temp;
    }




    // now print the reverse of the string
    printf("\nprefix of entered (infix) expression is:\n %s\n", pref);
    // getch();  // win not work on linux

}

int push(char *s, int top, char ele)
{

    int i;
    if (top >= MAX)
    {

        printf("\nStack Overflow");
    }
    else
    {
        s[++top] = ele;
    }

    return top;
}

int pop(char *a, int *top)
{

    if ((*top) >= 0)
    {
        (*top) = (*top) - 1;

        return a[(*top) + 1];
    }
    else
    {

        printf("Stack underflow\n");
        return 0;
    }
}

int pre(char x)
{

    int a;
    switch (x)
    {
    case '+':

    case '-':
        a = 2;
        break;

    case '/':
    case '*':
    case '\\':

    case '%':
        a = 5;
        break;

    case '$':
    case '^':
        a = 10;

        break;
    }
    return a;
}